The present invention relates to a process for manufacturing a multilayer printed wiring board or package with a cavity for mounting an electronic device such as a semiconductor element.
In manufacturing such a wiring board, a process is known in which a copper clad laminate or printed wiring board (PWB) with a hole is bonded to a printed wiring board without a hole by use of a prepreg with a hole as a bonding layer. With the prepreg mounted between these two boards in alignment with them, heat and pressure are applied to the assembly to melt the prepreg and bond the two boards together.
The known process had a problem in that the prepreg made of resin tended to melt and flow into the cavity for mounting a semiconductor element so as to cover the bonding pads formed on the lower board. This results in poor bonding with the mounted semiconductor element and thus poor electrical conduction.
One solution to this problem is to use a prepreg of a low-flow or non-flow type to minimize the possibility of molten resin flowing into the cavity. But, this might form a void or discontinuity on the wiring board.
Another solution is to use a prepreg with a larger hole than that of the upper laminate or board. Although this can prevent the molten resin from flowing into the cavity, a sufficient rigidity can not be obtained for secure bonding due to the formation of a gap between the bonding pads and the laminate or board with a hole. This results in poor bonding between the bonding pads on the lower board (PWB) and the semiconductor element.
FIG. 7 shows how the bonding pads can become covered with molten resin of a prepreg 23 which has flowed into the cavity.